Multi-layer golf balls contain a core, which may include one or more layers of solid material or one or more layers of solid material encompassing a liquid therein, and a cover. Optionally, an elastic winding may also be used to form a layer surrounding the center to provide certain playing characteristics. Such balls are known as "wound" balls. The multi-layer golf balls discussed herein include a core and a cover. The terms "core" or "ball core," as used herein, include a center having one or more layers and a mantle formed of one or more layers. The terms "center" or "ball center," as used herein, include a solid and/or liquid mass around which at least a mantle and cover are placed. The mantle is disposed between the center and the cover, typically in concentric fashion, with the cover being the outermost portion of the ball.
A variety of golf ball compositions are known and used in various methods of manufacture. Unfortunately, these compositions and methods tend to produce balls that do not consistently achieve a symmetrical core. See, for example, the discussion in co-pending Application No. 08/943,932 by J. DALTON et al., which illustrates the poor centering that occurs in conventionally formed golf balls. This co-pending application is expressly incorporated herein by reference thereto for this purpose. Multi-layer ball production has been plagued by center portions that become off-centered during the manufacture of such balls. Off-center golf balls are a hindrance to many players, particularly those able to achieve great control using a symmetrical ball. This lack of symmetry is now believed to be caused, at least in part, by the materials and methods conventionally used in forming multi-layer golf balls. A number of these conventional multi-layer ball compositions are discussed below.
U.S. Pat. No. 4,781,383 discloses a solid three-piece golf ball made by covering a core, which has inner and outer layers, with a shell. The outer layer of cis-1,4-polybutadiene, zinc diacrylate, and zinc oxide is prepared by using a metal mold to prepare two hemispherical premolded products, which are used to cover the previously molded inner layer of the core. The outer layer is then cured around the inner layer by heating the entire core before adding the shell.
U.S. Pat. No. 4,919,434 discloses a two-piece golf ball having a solid core of more than 40% cis-1,4-polybutadiene and a cover having an inner layer of 0.1 to 2 mm thickness and an outer layer of 0.1 to 1.5 mm thickness. The inner layer is a thermoplastic resin, such as an ionomer, polyester elastomer, polyamide elastomer, thermoplastic urethane elastomer, propylene-butadiene copolymer, 1,2-polybutadiene, polybutene-1, and styrene-butadiene block copolymer, either individually or in combination.
U.S. Pat. No. 5,150,905 discloses a rubber composition usable in golf balls having at least one natural or synthetic rubber component, inorganic fibers subjected to surface treatment, and a non-sulfur type vulcanizing agent. The rubber may include known additives, such as organic modifiers of various resins like cumarone-indene, phenol, polystyrene, acrylic, polyamide, epoxy, urethane, polyolefin, and similar resins. The rubber may also include long fiber reinforcing material, such as fibers of glass, carbon, metal, quartz, ceramic, nylon, vinyl, polyester, aromatic polyamide, polyimide, and aromatic polyether amide.
U.S. Pat. No. 5,253,871 discloses a three-part golf ball including an elastomer core, an intermediate layer of a thermoplastic material containing at least 10%, preferably at least 35%, of ether block copolymer, and a thermoplastic envelope. The other copolymer of the intermediate layer is disclosed to be one or more ionomers.
U.S. Pat. No. 5,314,187 discloses a golf ball having a core, as well as a cover having an inner layer of a cut-resistant material such as an ionomer resin and an outer layer of natural or synthetic balata and one or more thermally crosslinkable elastomeric polymers.
U.S. Pat. No. 5,439,227 discloses a multi-piece solid golf ball having a solid core with an inner layer of a rubber and an outer layer of 100-50 wt % of a polyether ester type thermoplastic elastomer having a T.sub.g of up to -25.degree. C. and 0-50 wt % of an ethylene-(meth)acrylate copolymer ionomer, and a cover of ethylene-(meth)acrylate copolymer ionomer.
U.S. Pat. Nos. 5,553,852 and 5,556,098 disclose a three-piece solid golf ball with a conventional rubber center core, an intermediate layer of thermoplastic elastomer or thermoplastic elastomer and ionomer resin mixture, and a cover typically of an ionomer resin, each portion having a particular hardness and thickness.
U.S. Pat. No. 5,601,502 discloses a three-piece solid golf ball including a core of a center having an .alpha.,.beta.-unsaturated carboxylic acid metallic salt in an amount of 13 to 28 parts by weight based on 100 parts by weight of base rubber and an outer shell having an .alpha.,.beta.-unsaturated carboxylic acid metallic salt in an amount of 28 to 35 parts by weight based on 100 parts by weight of base rubber. The base rubber preferably has a cis-1,4 structure of 40% or more, particularly 85% or more.
U.S. Pat. No. 5,681,898 discloses a golf ball having a solid core and a cover, with an intermediate layer including a first component of an uncrosslinked blend of n-butyl acrylate and ethylene methacrylic acid copolymer, which is sold under the name NUCREL, and a second component of a vulcanizate formed from polybutadiene and a peroxide curing agent. The vulcanizate is ground to a fine powder and then conventionally mixed with pellets of the NUCREL and melted for injection molding.
U.S. Pat. No. 5,683,312 discloses a golf ball having a fluid mass at the center, a first non-wound mantle layer of a thermoset rubber material, thermoplastic elastomeric material and plastic, a second non-wound mantle layer of a thermoset rubber material or thermoplastic elastomeric material, and a cover.
U.S. Pat. No. 5,688,191 discloses a multi-layer golf ball having a core with one or more layers, at least one cover layer, and one or more mantle layers disposed therebetween, wherein the mantle layer includes dynamically vulcanized thermoplastic elastomer, functionalized styrene-butadiene elastomer, thermoplastic polyurethane, metallocene polymer or blends thereof, and thermoset materials.
It is desirable to use thermoset material-containing hemispherical shells to form one or more mantle layers about a golf ball center, although this often results in poor centering of the mantle and other difficulties because thermoset materials are difficult to work with before they have been crosslinked. The polymers typically used in such shells tend to have a memory that urges the polymer back to its earlier or original shape, which necessitates rapid compression molding to crosslink the polymer as soon as the shells are formed. Hemispherical shells are also prone to trapping air in the apex of the mold cavities in which they are placed to assemble golf ball cores. Hemispherical shells also do not tend to readily fit within mold cavities having off-center parting lines, which also causes problems due to poor centering when forming golf ball cores.
There is thus a need for an improved composition and method for manufacturing golf balls that avoids the disadvantages present when using thermoset material-containing hemispherical shells to form one or more mantle layers about a center. A new composition for one or more layers of a golf ball mantle, and a method for manufacturing a portion of a golf ball core using ellipsoidal shells around a center, advantageously improves the symmetrical formation of the core in golf balls in accordance with the present invention. The proposed compositions facilitate injection molding of the uncrosslinked shells and permit automated assembly, which greatly reduces production costs.